The emergence of fuel cell technology has created a new tool for the generation of clean, high efficiency alternative energy for humans. The research and development of new catalysts to replace the expensive and rare platinum (Pt) to reduce the overall cost of fuel cells is ongoing in this area. Nitrogen-doped carbon and its composites possess great potential for fuel cell catalyst applications especially at the oxygen reduction cathode. It is proposed that the reaction mechanisms of nitrogen-doped carbon catalysts for oxygen reduction involve adsorption of oxygen at the partially polarized carbon atoms adjacent to the nitrogen dopants, different from the mechanism at platinum catalysts, which utilize d-bands filling at oxygen adsorption sites. Nitrogen doping in both carbon nanostructures and its composites with active metals or ceramics are reviewed. Nitrogen-doped carbon without composite metals, displays high catalytic activity in alkaline fuel cells and exhibits significant activity in proton exchange membrane fuel cells and direct methanol fuel cells. Pt-based catalysts with nitrogen-doped carbon supports show enhanced catalytic activity towards oxygen reduction, attributed to the enhanced anchoring of Pt to the support that results in better dispersion and stability of the electrodes. For nitrogen-doped carbon composites with non-noble metals (Fe, Co, etc) enhanced activity is seen in both proton exchange and alkaline fuel cells.
The oxygen reduction reaction (ORR) in fuel cell is a critical step and indeed a limiting factor for the widespread application of low temperature PEMFC. Pt based electrocatalysts are the most extensively used materials in fuel cell cathodes due to the sluggish kinetics of ORR. However the high cost, poor durability and limited availability of platinum render the device with several cost and performance issues. It is with these concerns that intense research on the development of alternative electrocatalysts is being currently carried out. Materials like transition-metal containing macromolecules, metal oxides/nitrides/oxynitrides and their composites with carbon have been demonstrated as effective ORR catalysts but with performance and stability still much lower than platinum.
Amongst the various options under investigation, nitrogen doped carbon materials are perhaps the best performing, inexpensive, highly stable, environmentally-friendly, metal free alternatives to the Pt based systems. There are several reports on the ORR performance of N-doped carbon nanofibers, N-doped Carbon nanotubes (CNTs), N-doped graphene, N-doped mesoporous carbons etc. These materials have been synthesized by in-situ (addition of an organic compound in the precursor) or post-synthetic techniques (e.g. post synthetic annealing in NH3 atmosphere etc.). Established synthetic routes towards such hetero-atom doped carbon systems include chemical vapor deposition, high temperature annealing, solvothermal synthesis, pyrolysis of organic compounds/polymers etc. In spite of much work in this area, most of these catalysts, including graphene based systems show ineffective catalytic performance and necessitate tedious synthetic protocols and expensive techniques/precursors, thereby restricting their applicability only to the small scale.
Recently, several interesting approaches have been adopted for the synthesis of highly effective doped carbon/composites as described below:
Article Titled “Biomass-derived activated carbon as high-performance non-precious electrocatalyst for oxygen reduction” by K Wang published in RSC Adv., 2013, 3, pp 12039-12042 reports a new type of Fe and N doped carbon material is synthesized by pyrolyzing ferric chloride doped egg white (EW) and the proposed synthetic route is easy, green, and low-cost. In addition, the as-prepared sample exhibits a feasible magnetism and comparable oxygen reduction reaction (ORR) activity to commercial Pt/C.
Article Titled “Microwave-assisted rapid green synthesis of photoluminescent carbon nanodots from flour and their applications for sensitive and selective detection of mercury (II) ions” by X Qin et al. published in Sensors and Actuators B: Chemical, 31 Jul. 2013, Volume 184, Pages 156-162 reports the microwave-assisted rapid green synthesis of photoluminescent carbon nanodots (C-dots) with diameters in the range of 1-4 nm using flour as the carbon source. It suggests that the resultant C-dots exhibit high sensitivity and selectivity toward Hg2+ with a detection limit as low as 0.5 nM and a linear range of 0.0005-0.01 μM. The practical use of this system for Hg2+ determination in real lake water samples is also demonstrated successfully.
Article Titled “From biomass wastes to large-area, high-quality, N-doped graphene: catalyst-free carbonization of chitosan coatings on arbitrary substrates” by A Primo et al. published in Chem Commun (Camb), 2012 Sep. 25; 48(74):9254-6 reports synthesis of N-doped graphene via pyrolysis of chitosan films under argon at 800° C. and under inert atmosphere gives rise to high-quality single layer N-doped graphene films (over 99% transmittance) as evidenced by XPS, Raman spectroscopy, and Transmission Electron Microscope (TEM) imaging.
Article Titled “One-step scalable preparation of N-doped nanoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction” by Z Liu et al. published in Nano Research, April 2013, Volume 6, Issue 4, pp 293-301 reports the N-doped porous carbon materials have been prepared by a simple one-step pyrolysis of ethylenediaminetetraacetic acid (EDTA) and melamine in the presence of KOH and Co(NO3)2.6H2O. The combination of the high specific area (1485 m2·g−1), high nitrogen content (10.8%) and suitable graphitic degree results in catalysts exhibiting high activity (with onset and half-wave potentials of 0.88 and 0.79 V vs the reversible hydrogen electrode (RHE), respectively) and four-electron selectivity for the oxygen reduction reaction (ORR) in alkaline medium—comparable to a commercial Pt/C catalyst, but far exceeding Pt/C in stability and durability. Owing to their superb ORR performance, low cost and facile preparation, the catalysts have great potential applications in fuel cells, metal-air batteries, and ORR-related electrochemical industries.
The most important challenge in fuel cells remains the synthesis of high-performance and cost-effective catalytic replacements for ORR that can be manufactured potentially on a large scale for a widespread deployment of fuel cell technology. The urgent need for sustainable energy development depends on the progress of green technologies, which have steered hot research areas into environmentally benign approaches via inexpensive precursors and abundant resources obtained directly from nature for energy devices such as fuel cells and supercapacitors.